Human ceramide kinase demonstrates phosphorylating activity against ceramide type sphingolipids. The activity is dependent upon the length of the fatty acyl moiety, with longer chain ceramides generally being phosphorylated at a higher rate than shorter chain ceramides. As chain length increases and ceramides become increasingly more difficult to solubilize, however, phosphorylation activity decreases along with the decrease in solubility. The difference in phosphorylation rates between the long and short chain ceramides does not appear to be due to slower dissociation rates for the shorter chain ceramides since mixing shorter chain ceramides with longer chain ceramides (for example C2 ceramide with C8 ceramide; or C0 ceramide (sphingosine) with C2 ceramide) does not appear to significantly affect phosphorylation rates of the longer species.
Ceramide, the central molecule in the sphingomyelin pathway, serves as a second messenger for cellular functions ranging from differentiation to growth arrest and apoptosis. Ceramide signaling contributes to several aspects of immune cell function. It is a second messenger for the cytokine TNFα, IL-1β, IFNγ, and has been implicated in the signaling pathways of several lymphocyte surface proteins. Following its generation, ceramide can be converted to sphingosine, phosphorylated to ceramide-1-phosphate, converted to sphingomyelin, or glycosylated to form glycosphingolipids. It can also act without conversion by interacting directly with proteins downstream in the signaling cascade. One of the most well studied downstream effects of ceramide signaling is apoptosis. Ceramide is an important mediator of many of the cell surface receptors involved in transmitting death signals into cells, particularly receptors of the TNF receptor superfamily.
Possibly as a negative feedback mechanism to counter ceramide-mediated cell death, sphingosine-1-phosphate, generated after conversion of ceramide to sphingosine by ceramidase and then phosphorylation of sphingosine by sphingosine kinase, can promote cell survival. Sphingosine-1-phosphate released by cells can act as a potent agonist of cell-surface receptors of the EDG receptor family, among others, and prevent cell death initiated by a various apoptosis-inducing treatments. Sphingosine-1 phosphate has also been reported to act as a growth and differentiation factor.
There is a need in the art for identifying new ceramide kinase proteins and methods of regulating intracellular signaling and apoptosis.